The present invention is directed to a rock anchor assembly for securing roadways and the wall surfaces of open cuts and tunnels. The anchor bolt assembly is formed of an axially elongated tension member laterally enclosed at least along a portion of its length within a sheathing tube. The leading end of the tension member is anchored within the base of the borehole and the trailing end of the sheathing tube is pressed against an anchor plate abutting against the surface of the receiving material in which the borehole is formed. The sheathing tube extends along a considerable part of the tension member within the borehole and, at its leading end, the sheathing tube is in positive locked engagement with the tension member at a location adjacent but spaced from the leading end of the tension member.
In a known rock anchor assembly of this type, a steel sheathing tube is bonded with the tension member over the length of the anchoring distance at the end of the tension member located within the borehole so that the tension member extends from the end of the sheathing tube with only a short mixing tip, note DE-PS 34 17 252. To assure that the sheathing tube fractures at a specific location in the event of an overload, a rated failure location is provided in the sheathing tube at the end of the anchored distance. The remainder of the sheathing tube extending to the location where the tension member is anchored on the outside of the borehole is in bonded connection with a hardenable synthetic resin adhesive material which fills the borehole.
In this known rock anchor assembly, the sheathing tube, which has a surface configuration for enhancing the bond with the adhesive material, acts as a rigid anchor. As soon as forces are developed due to displacement of a rock formation exceeding the load capacity of the sheathing tube, the tube is intended to fracture at the rated failure location, so that the extensibility of the tension member located within the sheathing tube becomes active. Accordingly, it is possible to take advantage with one anchoring assembly of a rigid and then an extensible anchor member.
In another development of the known rock anchor assembly, the sheathing tube is made up of at least three sections connected together by rated fracture locations in a frictionally locking manner, with only the sections arranged at the opening into the borehole and at the base of the borehole provided with a profiled configuration for enhancing bonding with the adhesive. Between the profiled surfaces, the sheathing tube has a smooth surface section, note DE-PS 35 31 393. This arrangement is intended to prevent the sheathing tube, embedded over its entire length into the hardenable synthetic resin adhesive filling the borehole, from possibly fracturing at the location of an opening in the rock formation if its load carrying capacity is exceeded and the forces are suddenly applied on the load yield characteristic of the tension member alone. Due to the smooth surface of the central section located between the end sections of the sheathing tube, which are in bonded connection with the adhesive material, and the arrangement of the rated fracture locations of the sections of the sheathing tube fastened to the tension member, it is intended that the sheathing tube fail at the rated fracture points if an opening develops in the rock formation so that an additional change in length occurs under the influence of friction with respect to the hardenable adhesive material surrounding the sheathing tube, which friction affords the compensation of the load yield curve.
The most dangerous stresses for such a rock anchor assembly are displacements in rock formations with respect to one another in a direction extending transversely of the axial direction of the tension member, which tend to shear the tension member. As a result, the maximum force acting on the tension member is located at the point of the shearing action and not at the anchorage. As the bond between the tension member and the hardenable synthetic resin adhesive material filling the borehole improves, then the shorter would be the effective length of the external forces acting on the tension member and the higher stresses arising in it. The disadvantage of this known rock anchor assembly is that, even if the surface of the sheathing tube is shaped along specific sections for increasing its bond, comparatively large lengths of the sheathing tube are required for anchoring it in the boreholes which are not available for its function especially with reference to displacement in the rock formation. This known anchor assembly is not effective if a rock formation displacement takes place in the region of one of the anchoring sections in the base of the borehole or near the opening into the borehole. Moreover, additional expense is involved in shaping the sheathing tube surface for improving its bonding action.